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Download - Hochschule München

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4.2.1 XRD Phase analysis

4.2.1 XRD Phase analysis When looking at the XRD tableau generally, we suspected for the thermo dynamic phase diagram (figure 2.8) the entire tableau to be in yellow (WC). As discussed in section 2.5 the only carbide phase that exists within samples with a tungsten concentration of 6%- 25%, annealed to temperatures from room temperature up to 2800K is WC. However, WC is only mainly found within the higher concentrated and annealed films. Besides the suspected WC, also W2C and WC1-x can be found within the lower concentrated films. WC1-x is observed in the low concentrated a-C:W films, whereas W2C is present in the intermediate concentrated a-C:W films. Besides the presence of all three carbides within the tableau, most of the samples contain a mixture of two carbides. From all specimen containing a mixture of crystallites, WC is always one of them. Only the low concentrated samples contain a mixture of WC and WC1-x after heat treatment to 2800K. All other samples contain a mixture of WC and W2C. Unlike with WC1-x, these mixed samples only appear after heat treatment from 1450K up to 2200K. Above 2200K, no more W2C is present, only WC. The disappearing of W2C can be explained by the fact that W2C is a transit-phase for WC as is discussed in the thesis of Jens Luthin. Both carbides have a hexagonal structure and WC is thermodynamically the most stable carbide. However, this does not define the formation of W2C, whose building enthalpy is higher than that of WC. In addition, the transport of tungsten necessary to build W2C requires more energy than that of carbon. The formation of WC1-x in the lower concentrated films 6-9 at.% W is very remarkable. WC1-x is known to be thermodynamically stable around temperatures of 1500K-2800K and at tungsten concentration levels of 50-62.9 at.% (figure 2.8). However in our case, WC1-x can be found around concentration levels of 6-10% and from 2200K up to 2800K. Nevertheless, since XRD is our primary characterization method, it could be possible that WC1-x exists at lower temperatures but is not detected. The formation of WC1-x seems strange due to the redundant amount of carbon present and that the building enthalpy of WC is lower, making it more thermodynamically stable. This would make a combination of both carbides more reasonable than just finding WC1-x in the annealed samples. In the 2800K samples, both WC and WC1-x can be spotted. Another interesting fact is that within the annealed samples of 1100K and 1300K, no clear carbide formation was found. The mobility in atoms in these films seems to be too low at these temperatures. - 40 -

Comparing the crystallite sizes of the W2C phase in all samples shows that the maximum crystal size does not exceed 20 nm. All samples which contain W2C have small crystal sizes, meaning that W2C can be considered to be meta-stable, in non-thermodynamic state. The maximum size of WC1-x crystals are larger than 60 nm, whereas the maximum size of WC crystals cannot be determined using XRD-methods, but are larger than 100nm. XRD Tableau (next page): The elements within the XRD Tableau have been divided into two sections. The left section, illustrates which carbide can be observed within the sample. Furthermore, it tells about the abundance of each carbide phase by comparing their peak intensities by the area fraction. The right section, divided into 3 rows where each row represents one of three carbides, gives us information about the grain sizes. The height of the row informs us about the average crystal size of the respective carbide within the sample calculated with the Scherrer equation (2.3). In the left upper corner of the tableau, a color index for is shown. On the right side of the Tableau two lines illustrate where which grain size is found. The grain sizes scale is divided into following sizes (from bottom to top): < 5, 5, 10, 20, 30, 40, 50, 60, 70, 80, 100 and 200 nm. At the bottom of each row, the average crystal size is given for better comparison. No textures were interpreted from any diffractogram during this research. SEM tableau (page 45): The elements in SEM/STEM tableau are also divided into two sections. The left section contains information acquired from nano-diffraction in Warsaw and gives us information about the carbide phases located within the sample as well as abundance. From the lamella, small crystallites were randomly chosen on which then nano-diffraction was performed. The right section is a schematic drawing of crystal sizes and size distribution acquired from SEM. The height of each row is a rough estimate symbolizing the quantity of crystals numbers in that size range in the sample. Both scales have been plotted in a roughly logarithmic scale, so that all sizes can be plotted. The scale used for the crystal sizes (from left to right) is divided into: 1, 2, 5, 7, 10, 20, 50, 100, 200, 500, 1000 and 2000nm. In the left upper corner a color index is illustrated. Two lines near the 2500K 18% sample, demonstrate where the next range begins. - 41 -

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